A Tale of Three Caterpillars (?)

Here's a fun shot from a few days ago:

Dotted Gray (Glena cribritaria),
Van Patten Woods FP, Lake Co, IL  3/30/2014

This little twig was, apparently, growing off of a wooden signpost in a local Forest Preserve. (I've photographed some neat little critters on those posts.)

Of course, closer examination proved it to be a caterpillar -- specifically (I think) a Dotted Gray (Glena cribrataria), one of the Geometridae. You can tell it's a Geometrid because it's only got three pairs of abdominal prolegs. (Those leg-like structures at the back end, including the pair holding onto the post.)

This sort of camouflage is quite common in caterpillars. After all, if you're slow, taste good (I assume they do, since warblers love them, but I think I'll forgo testing that), and don't have teeth, you're better off hiding.

So what about this guy?

Unexpected Cycnia (Cycnia inopinatus), Illinois Beach State Park,
Lake Co, IL 9/22/2011

This is an Unexpected Cycnia (Cycnia inopinatus). That bright color really stands out -- there were a lot of them around that season, and they were easy to see in the green vegetation. Clearly, they're not trying to hide.

The spines on the back might be part of the answer here. Some caterpillars have irritating hairs, and some can flat-out sting. (Saddleback Caterpillars are notorious for it!) But while it's hard to tell from the photo, the plant he's eating has more to do with that bright color. That's a Swamp Milkweed (Asclepias incarnata). Like all Milkweeds, it produces some rather nasty defensive compounds, which any insect that eats it is going to have to deal with. Most prefer to avoid eating milkweeds entirely, but those that will eat them often sequester those chemicals, making themselves distasteful to their potential predators. Of course, being distasteful isn't much help if the predator has to eat you to find out, so many of these species have developed bright colors as a warning to predators.

Frequently, when we find species that have those warning colors, we also find species that mimic them, gaining the protection without having to incur the costs. But sometimes species resemble each other for entirely different reasons. Like this guy:

Phymatocera sp., Van Patten Woods FP, Lake Co, IL 8/23/2013

This isn't a caterpillar at all -- it's a sawfly, in the genus Phymatocera. Sawflies are in the same order as bees and wasps, and many of the adults resemble wasps, but they can't sting. The larvae, as seen here, resemble caterpillars and mostly eat plant materials. (The easiest way to tell them apart is to count the prolegs. Caterpillars will have 5 pairs or fewer (as few as 2 in geometrids), sawflies have 6 or more.) But they aren't mimicking caterpillars -- this general body plan appears to be an ancestral larval state for a large group of insects, with beetle grubs, wasp larvae, and caddisfly larvae all looking like this as well.

Camouflage and mimicry have produced some amazing lies in the animal kingdom, but it's worth remembering that sometimes the mimicry itself is an illusion.